The present invention relates to a gear-changing appliance of a gearbox. In a known gear-changing appliance of the type described in DE 4124385 C1, the pressure regulating valve for the supply pressure operates as a function of a load-dependent it modulating pressure, for whose generation an independent regulating system is necessary. This system increases the costs of the electrohydraulic control and takes up valuable installation space within the conventionally employed control plate in the gearbox casing.
In the known arrangement of DE 42 41 593 A1 for automatically shifting an epicyclic gear-changing gearbox of a motor vehicle from a previous old gear, in which a first frictional connection (clutch or brake) is engaged by an associated hydraulic fluid gear-change actuator, into a new gear. A second frictional connection is engaged by an associated hydraulic fluid gear-change actuator, and the first frictional connection is disengaged by switching off the working pressure from the associated gear-change actuator. The working pressure of the gear-change actuator of the new gear is measured, and the working pressure of the gear-change actuator of the old gear is switched off if the working pressure of the gear-change actuator of the new gear reaches or exceeds a significant pressure value determined by a comparison with a reference parameter. In order to be able to switch off the gear-change actuator of the previous gear during a gear change, also taking account of gear-changing comfort and service life of the gear-changing means involved, the gear-changing comfort is possibly impaired by the gear-changing jolt which may occur and the service life is possibly impaired by excessively long slip times of the friction surfaces. Therefore provision is made in this known arrangement for the reference parameter to be variable as a function of the load torque on the driving engine, taking account, if necessary, of the converter relationship of a hydrodynamic torque converter and the significant pressure value is associated with that instantaneous value of the transmittable torque of the frictional connection of the new gear which corresponds to the instantaneous actual value of the load torque. In this known arrangement, the working pressure of the gear-changing actuators is adjusted by electromagnetic pressure regulating valves which can be activated by an electronic control unit, a load-dependent supply pressure being supplied to the pressure regulating valves. No statement is made in the publication with regard to the formation or generation of this supply pressure.
Finally, another type of device described is also described in DE 36 30 792 A1. In order to generate a main pressure dependent on the operating state of the motor vehicle for activating actuators of an automatic motor-vehicle gearbox, this device is provided with an adjustable delivery pump which adjusts the main pressure by changing the delivery quantity. A control valve is used to supply a pump adjustment pressure acting on a quantity adjustment device of the delivery pump in such a way as to adjust the delivery quantity. In order to ensure, using the simplest possible means, that the main pressure is optimally adjusted to correspond to the operating state of the motor vehicle in each case, a configuration is selected such that the control valve has a regulating piston which generates the pump adjustment pressure, and which is subjected on one balance surface to the fed-back pump adjustment pressure and on an oppositely-directed balance surface to a pressure which varies as a function of the operating state and the control piston acted on by the main pressure.
The present invention may be essentially considered as being based on the object of keeping the outlay on producing the load dependence of the supply pressure small in a gear-changing appliance. The object explained is advantageously achieved, in accordance with the invention, by deriving a control pressure from the working pressure to constitute a load-dependent control signal.
The switching appliance according to the invention is distinguished by the fact that only one frictional connection is engaged and is subjected to working pressure in each gear. The working pressures are directly regulated by electromagnetic pilot valves and associated pressure regulating valves. The working pressures can be freely regulated, as they are not interdependent.
The gear-changing appliance according to the invention is further distinguished by the fact that the six frictional connections or their actuators, necessary for example in the case of a six-speed gearbox, are-supplied with working pressure through the action of a total of only three pilot valves and their associated pressure regulating valves. The allocation of the regulated working pressures to the actuators can take place by a manual selection slide valve and two switching valves which are activated by a positional control valve. In the gear-changing appliance according to the invention, the actuators are exclusively subjected to pressure from the pressure regulating valves for the working pressure. There is no changeover to the supply pressure. The gear-changing appliance according to the invention is, furthermore, distinguished by the fact that only the working pressure of the frictional connection carrying power or of the associated actuator determines the respective necessary supply pressure.
In the gear-changing appliance according to the invention, the basic value of the supply pressure is fixed by a valve spring of the relevant pressure regulating valve when the gearbox is not in a power-transmitting condition.
An advantageous embodiment of the priority controller in the gear-changing appliance according to the invention is described herein.
A measure is directed toward a safety function of the gear changing appliance according to the invention, and this function avoids the pressure regulating valve for the supply pressure adjusting to an excessive pressure if a pressure regulating valve for the working pressure jams in transit position.